Active PA speaker issue

Moltisanti

One of my Active PA speakers was has an issue - it powers on OK but audio is only coming through the tweeter not the main speaker. There's nothing coming out of the large speaker at all, it doesn;t sound broken, there's just nothing there.

Apart from an obvious connection issue, is there anything else i can look for on my own ? 

it's a DB Technologies Cromo 12

cheers

ICBM

It will almost certainly either be a blown bass speaker or the power amp for it. If you can get into it - sometimes this is difficult - you could test the bass speaker.

Disconnect the wires (they will usually be push-connectors, make sure you note which goes where) and if you have a multimeter, meter across the terminals - if the speaker is OK you should get about 3 ohms or 6 ohms depending on if it's a 4 or 8-ohm speaker.

If you don't have a meter, touch a PP3 battery across the terminals - you may need a screwdriver to bridge the gap if they're far apart. If it makes a pop and the cone moves, the speaker is fine. Don't keep the battery connected for any longer than you need to.

If it's the speaker and you can get a direct replacement you can change it yourself. If it's the power amp, it may be beyond repair even by most techs, other than by simply replacing the whole power amp module - Class D power sections are notoriously difficult to troubleshoot and many are not designed to be easily worked on either.

jpfamps

If the speaker is blown I would be wary of simply changing it without testing the amplifier.

I've seen plenty of amps where the amp has latched up and blown the speaker, in which case changing the speaker will simply result in another blown speaker.

Equally the amp may have expired, and you will still have a non functioning amp but will have spent a load on a speaker (replacement 12" drivers for powered speakers are surprisingly expense when compared with the cost of the unit).

ICBM

Good points. I would test the amp independently if the speaker is dead, it’s easy to forget things which you do without thinking about...

Moltisanti

ok thanks for the input, either way a dead amp or dead speaker is probably not worth replacing given what i paid for it. I can still use the other one as a monitor

Moltisanti

If the internal amp is broken is it possible to bypass/remove it and still use the speakers with a separate power amp ?

ICBM

Moltisanti said:

If the internal amp is broken is it possible to bypass/remove it and still use the speakers with a separate power amp ?

Yes. I’ve done a couple of Wharfedale powered bass bins like that. Remove everything from the back panel and fit a jack in one of the spare holes. (Tape over the others from both sides so it sticks to itself.)

It will be more difficult with a full-range cab though - you will need a crossover as well.

Moltisanti

so i tested this speaker today, and now it's fine...so weird.

Problem is i can't rely on it as it wasn't working last time...

Any idea as to what could cause an intermittent problem like this, i'm a bit baffled...

p90fool

Could be something as simple as an internal connector coming loose, would be worth opening it up and checking. 

ICBM

Moltisanti said:

so i tested this speaker today, and now it's fine...so weird.

Problem is i can't rely on it as it wasn't working last time...

Any idea as to what could cause an intermittent problem like this, i'm a bit baffled...

If it's not a connector, the two most common causes of this I've come across are a faulty speaker voice coil - the solder joint were it joins the braided wire to the terminal - only known for certain by 'post-mortem' once the speaker was definitely dead, not repairable; and a cracked PCB solder joint on the power amp board, which should be fixable - if you can get at it - but can be very hard to detect.

You're right, you can't rely on it.

Danny1969

Cromo 12 has 2 class D amps in it, one for bass driver and one for horn and some clever active crossover, like they all do now. 
If the horn didn't cut out then you can ignore everything primary of the transformer and the switching and pulse width circuit as any failure there would kill both amps. 

Bad joint on the actual power amp that feeds the woofer or a bad spade connector would be my best guess. Have a good visual inspection ... I've seen MOV legs and thermistors legs break to the vibration in active speakers ... it's good form to gunk em but some don't bother 

Moltisanti

just opened both up and everything seemed solid, nothing loose or mis-aligned, took a couple of pics as i've seen inside one of these before

Danny1969

The PSU is in the speaker still by the looks of things, I would wager the 5 connector with the black in the middle is the dual power with the black at 0V
Were you driving the speaker hard when it cut out, it's possible thermal protection has cut in when amp got too hot OR thermal protection is faulty causing amp to turn off

Not an easy thing to diagnose online and sometimes not an easy thing to fix on the bench if it's the first one you've seen. Class D amps are complex in operation and repairs (in my experience) generally require sketching bits of the circuit out and working from there if there's nothing obvious 

Moltisanti

thanks for the advice all, mystery faults are the worst! 

ICBM

Also check the speaker terminals - I can't quite see clearly enough in the pic, but if the cone braids are crimped to the terminals with a little folded metal clamp, these can get corrosion in and stop working - I've found this often enough that I now solder them up as a general maintenance job whenever working on any speaker that has them.

Any crimped or push contact connector is still the prime suspect.

Moltisanti

i checked the spade connectors and they looked clean and were on solidly. I would have had to remove the whole housing to get a soldering iron in there, but i might do it at a later date. Thanks for the tip